Radio broadcasting system



May 2.6, 1931. R. BowN RADIO BROADCASTING SYSTEM Filed Dec.

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b SSE@ INVENTOR mz/I/a BY ATTORNEY Patented May 26,1931

RALPHBOWN, F MAPLEVTOOD,

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NEW' JERSEY, ASSIGNOR TO .AMERICAN TELEPHONE AND TELE-.GRAPH COMPANY, VA CQRPORATION OF NEW YORK RADIO .BROADCASTING- SYSTEM Application filed December 111 1928. Serial No. 325,151.

This invention relates to radio broadcasting systems, and more particularly to a method of and means for reducing interference between broadcasting stations transmitting simultaneously a common program and all occupying the same carrier frequency channel assignment'. y

In order to mitigate congestion in the broadcasting field, it has at times been con-' the fact that it will tend to materially reduceV interference in systems of this type without there being necessity for synchronization of the carrier frequencies. y

The arrangements of the present invention propose utilizing a system combining a plurality of radio volume compression stations operating to broadcast a common program on a common frequency.YYY The method of operating these volume compression stations involves changing their carriersin aocordance with the changing volume of the common programV being broadcast.V The volume compression type of system vor the carrier amplitude control type of system `is well known in the part. An illustration of one form thereof may be found in U. S. Patent No. 1,681,972, issued to O. B. Blackwell, et al., on August 28, 1928.

The use of the volume compression type of system, in accordance with the arrangements of the invention, will afford certain interference reducing properties. The interference action of the carrier of the station being rereducing property resides inthe fact that the ceived and the carrier of the station which is looked upon as interfering, the loudness of the beat note is proportional to the product of the amplitude of these two carriers. The fact that the carrier amplitude is reduced during the weaker, parts of the program as transmitted from'a volume compression station causes the beat note amplitude between it and any other station to be decreased during the weak parts of thepro- `gram and the interfering effect thereby materially reduced. lt is evident that if the two stations under consideration were both transmitting the same program and were both volume compression stations, both carriers would be reduced in amplitude during the weak parts of the program and the interference between them would be doubly decreased. Other features and objects of the invention will appear more fully from the detailed description thereof hereinafter given. Y Y The invention may be more fully understood from the following description, to-

gether with the accompanying drawings, in the Figures 1 and 2 of which the invention is illustrated. In Fig. 1 is a circuit diagram showing avpreferredform of the invention. In Fig. 2 is shown a ymodification thereof. Like reference characters have been utilized to denote like parts in both of the figures.

In Fig. 1 is shown a radio broadcasting system comprising a pluralityof tran-smit-V ting stations, such as stations A, B and C. These stations will all be simultaneously broadcasting a common program originating from the studio 1. In order to relieve congestion in the broadcasting field, all of these stations will be operating on the same carrier frequency channel. Station A will be connected to the studio by circuit l2 and will be arranged to operate as aivolume compression system. Circuit 2 is connected with theV modulator 5 by the transformer 29, the con-V nection including a network 16 which may be designated a delay circuit, the function of which is to retard the signal from the studio which isV about to `be impressed upon the Ymodulator 5 and to prevent its effect thereon until certain other operations have taken place. Bridged across circuit 2 at the points 17 is a carrier amplitude control circuit comprising a rectifier 19, a low-pass filter 20, and a push-pull modulator 23 haring a carrier frequency generator which is connected with the input side thereof. This carrier amplitude control circuit is connected iith circuit 2 by means of the transformer 18, and the output side of this control circuit is connected with the input circuit of the modulator 5 by the transformer 2li, the connection also including` a filter or tuned circuit 25 which transmits substantially only the carrier frequency generated by the source 6. rlhe output side of the modulator 5 is connccted by the transformer 2T With a carrier circuit Which may contain an amplifier and filter, such as 28, which may be connected to an antenna for broadcasting.

The manner in which the before-described circuit functions to transmit speech, or other signals, is a follows: A portion of the speech energy from the studio 1 is impressed at the points 17 across the primary of transformer 18. The tube 19, which is arranged to function as a rectifier, rectifies the signal Wave, and the resultant rectified current is smoothed out by means of the network 20 constituting a low-pass filter so that only its major variations are transmitted. This rectified current, as smoothed out by the low-pass filter 20, flows through the resistances 21 and 22 in series across the grids of the push-pull modulator 23. The modulator grid is accordingly unbalanced so that a Wave of carrier frequency from the source 6 will be impressed upon `the secondary Winding of the tra-nsformer 24, the amplitudeof which will depend upon the degree of imbalance of the said modulator. This in turn depends upon the magnitude of the rectified current flowingthrough the resistances 21 and 22 in series, which establishes a potential difference across them with variations in the magnitude of the signal current. The relation between the amplitude of tue carrier and that of the signal input may or may not be arranged to be exactly linear, as desired. 'if ie filter 25 is designed to pass only slow variations of the carrier frequency, thus eliminating any resid.- nai side band components that might be produced by signal variations in the rectied signal frequency not eliminated by loW-pass filter 20. This carrier frequency will be .modulated by the signal frequency impressed by the transformer 29 upon the modulator 5, and the resultant side bands and the carrier frequency Willbe impressed by the transformer 27 upon the antenna circuit. A filter 28 may be included to eliminate one of the side bands for transmission, and an amplifier may also be inserted in the antenna circuit. The ldelay circuit 1G, by retarding the speech signal from the studio, enables the change in the amplitude of the carrier current to be effected before the signal reaches the input of the modulator 5. lt will accordingly be seen that as the speech, or other signal, diminishes in intensity, the amplitude of the carrier is likewise reduced, and, by proper adjustment of the rectifier, entire suppression of the car- 1ier in so far as it affects the input of the modulator 5 may be effected When there is a pause or cessation of the signal from the studio.

Other stations, such as B and C, may be connected by a common circuit 3 to the circuit 2 and thence to the studio. Station B Will include transmitter apparatus 8 and a carrier amplitude control circuit 7. Station C will include transmi :ter apparatus 10 and a carrier amplitude control circuit 9. ln detail, the arrangements at stations B and C will be subst ntially identical With those heretofore described with respect to station A, and hence the arrangements of stations B and C have been shown in schematic form only. lt is pointed out that the carrier frequency generated at stations B and C Will be the same as the carrier frequency produced by the generator G at station A.

As has been pointed out previously, beat note interference results from the interaction of the carrier of the station being received and the carrier of the station which is looked upon as interfering. Accordingly, the loudness of the beat note is proportional to the product of the amplitude of these tivo carriers. The fact that the carrier amplitude is reouced during the Wea-lier parts of the program, as transmitted from a volume corn- 'pression station, causes thebeat note amplitude between it and any other station to be decreased during the Weak parts of the program, and the interfering effect is thereby materially reduced. `When all of the stations under consideration are transmitting the same program, and are of the volume compression type of station, it is evident that the carriers would all be reduced in amplitude during' the weak parts of the program. and the interference between any two thereof would be further decreased.

A noditication ofthe system shown in Fig. 1 is illustrated in Fig. 2. Similar reference characters have been utilized to denote like parts in both figures. Stations A., B and C are shown having transmitter apparatus, such as l, 8 and 10. rlhese transmitters may be connected to studio 1 by the circuits 11 and 13. llt. a station, such as A, there will be provided the carrier amplitude control apparatus 12. By means of circuits lll, 15 and 35, this carrier amplitude control apparatus may be made common to the transmitters of all of the stations. The apparatus Will function in a manner substantially similar to that heretofore described.

While the invention has been disclosed as embodied in certain specific arrangements method Y of which are deemed desirable, it is understood that it is capable of embodiment in many other widely varied forms. It is particularly pointed out that apparatus other than the type shown may be utilized for controlling the carrier amplitudes. Accordingly, the arrangements of the'invention may be embodied in other forms without departing' from the spirit of the invention, as defined by the appended claims.

What is claimed is:

l. In a radio broadcasting system the simultaneously transmitting from a plurality of stations each operating on substantially the same carrier frequency a set of signals from a single source, which comprises simultaneously causing the am-V plitudes of the unmodulated` carrier frequency components vtransmitted from a plurality of said stations to vary with the amplitude of said set of signals, whereby the detected beat note resulting from two slightly different carriers will be the product of two carriers each of abnormally small amplitude when the amplitude of the desired signal is small. v

2. In a radio broadcasting system the method of signaling which consists in generating voice frequency signals at one source,-

said circuits for simultaneously controlling the amplitudes of the unmodulated carrier frequency components as transmitted from said transmitters in accordance with the am-V plitude of said low frequency signals, means to receive the modulated carriers to reproduce said low frequency signals, said means cember, 1928.

RALPH BOWN.

when the amplitude of the desired signal is Y small.

3. Ina signaling system the 'combination with a single source of low frequency sig'- nals, of a plurality of sources of substantially the same carrier frequency, means for simultaneously controlling the'amplitudes of said carrier frequencies in accordance with the envelope ofthe low frequency signals, means for simultanenously modulating the controlled carrier frequencies by the low frequency signals for broadcasting purposes, means to receive the modulated carriers to reproduce said low frequency signals, said means producing a detected beat note from two slightly different carriers which is the product of two abnormally small carrier components when the amplitude of the desired signal is small.

4. A. source of low frequency signals, a plurality of radio transmitters each operating on substantially the same carrier frequency, circuits for connecting each of said radio transmitters to said source so that the correspending carriers will be modulated by said low frequency signals, means associated with 

